We have shown previously that administration of prolactin to hypophysectomized mice and rats can increase testosterone secretion. Treatment with inhibitors of pituitary prolactin release decreases the concentration of esterified cholesterol in the testis and the weight of the seminal vesicles. We will study the effects of prolactin deficiency induced in normal intact males on plasma testosterone levels, spermatogenesis, testicular cholesterol stores, and the weight and secretory activity of accessory reproductive glands. Prolactin deficiency will be induced by treatment with 2-bromo-alpha-ergokryptine (CB-154), by immunization with heterologous prolactin and by injections of anti-prolactin serum. The results will indicate whether prolactin has a physiological role in regulating the functions of the male reproductive system. Some of these experiments will be done in guinea pigs and in rams to test if the influence of prolactin on the testis is limited to mice and rats. We will also study the effects of prolactin and LH on the activity of enzymes regulating the turnover of cholesterol esters in the testis. This will provide information as to whether the increase in testosterone secretion after prolactin administration is related to increased availability of cholesterol for steroidogenesis. In addition, we will examine the relationship of testosterone concentration in the rete testis fluid to that in the peripheral circulation.